1. Field of the Invention
The present invention relates to a system for controlling the oxygen concentration in exhaust gas and an exhaust gas cleaning system employing it, in which an adsorbent is provided either in an exhaust gas cleaning catalyzer provided in the exhaust passage of the engine or in the exhaust passage upstream from the catalyzer so that the adsorbent adsorbs unburned hydrocarbons, which are emitted in large amounts immediately after the engine is started, and releases the unburned hydrocarbons when the catalyzer has been heated to a temperature at which it becomes able to clean the exhaust gas.
2. Description of the Related Art
Generally, an engine is provided with a catalyzer connected to the exhaust passage so as to clean the exhaust gas. Such a catalyzer, however, becomes able to function only when it is heated to about 300#C or higher. Therefore, when the engine is just started, i.e., when the temperature of exhaust gas is not high enough, the catalyzer does not fully work to clean the exhaust gas. Moreover, an engine which is just started and still cold emits a lot of unburned hydrocarbons. Methods to solve such a problem are disclosed in Japanese Utility Model Unexamined Publication No. 60-190923, Japanese Patent Unexamined Publication Nos. 63-68713, 2-75327 and 2-135126, in which methods an adsorbent is used to temporarily adsorb unburned hydrocarbons and release them when the temperature of the exhaust gas rises to a sufficiently high level so that the catalyzer can fully work to clean the exhaust gas.
However, though Japanese Utility Model Unexamined Publication No. 60-190923 and Japanese Patent Unexamined Publication No. 2-75327 describe that a switch valve, which is provided in the connecting portion between the main passage and the by-pass passage provided with an adsorbent, is operated in accordance with the temperature of exhaust gas, they do not refer to treatment of the unburned hydrocarbons released from the adsorbent. Japanese Patent Unexamined Publication No. 2-135126 does not refer to controlling. Though Japanese Patent Unexamined Publication No. 63-68713 describes that a switch valve, which is provided in the connection portion between the main passage and the by-pass passage provided with an adsorbent, is operated in accordance with the temperature of exhaust gas and that a secondary supply of air is provided at least when the temperature of the adsorbent is as high as or higher than the level at which the adsorbent releases the unburned hydrocarbons, it does not refer to a method of controlling the secondary supplying of air. Further, while this Japanese Patent Publication describes that the adsorbent is cooled, for example, by using the air conditioner installed in the motor vehicle, it does not describe the control method for cooling the adsorbent.
Though a method is known in which an adsorbent is used to temporarily adsorb unburned hydrocarbons and release them when the temperature of exhaust gas becomes sufficiently high so that the catalyzer can work to clean the exhaust gas, only Japanese Patent Unexamined Publication No. 63-68713 describes how to efficiently convert the released unburned hydrocarbons by using the catalyzer, and even this specification merely tells that a secondary supply of air is provided when the temperature has reached the level at which the unburned hydrocarbons are released.